Radio Frequency Identification (“RFID”) tags are finding wide application in inventory control and tracking systems. In these systems, a two-way communications link is established wherein readers (or interrogators) interrogate RFID tags to respond with their identification and perhaps other ancillary information. The RFID tags can be broadly separated into three categories, namely, active, semi-active and passive. The active RFID tags contain a battery and communicate on the reverse link (RFID tag to reader) using conventional radio frequency (“RF”) communications techniques. The passive RFID tags typically derive power from the reader using a diode rectifier and then respond on the reverse link using a backscatter modulation technique that modulates the apparent radar cross section of the device. The semi-active RFID tags employs properties from the other two in that it has a battery, allowing it to respond to signals of lower amplitude than a passive RFID tag, however, its response is the same as a passive RFID tag in that it employs a backscatter modulation technique. The most common backscatter modulation approach is to modulate by shorting/opening an antenna of the RFID tag to vary the radar cross section (“RCS”).
One major weakness with extant RFID systems is their poor sensitivity limiting operation to fairly high signal-to-noise (“SNR”) regimes. This problem has been addressed, in part, with major improvements in sensitivity using knowledge of the target RFID tag's identification and a combination of coherent and noncoherent integration techniques. As addressed herein, further improvements are necessary to more accurately localize the position of an RFID tag.